184 GENETICS: J. LEDERBERG Proc. N. A. a 



example, a Laci — mutation occurring inaT — L — Bi— stock has been trans- 

 ferred to a combination with B — M — , allowing it to be compared genetically 

 in crosses with other Lac— mutants induced in T — L — B : — material. 



The experimental production and maintenance of diploids raises ques- 

 tions of dominance and dosage effects. It has already been noted that 

 the normal alleles of several biochemical mutations are dominant, although 

 quantitative comparisons of enzyme competence in heterozygotes, haploids 

 and homozygotes remain to be carried out. The dominance of the type 

 sensitive alleles of genes for phage resistance supports, at least in part, a 

 segregation interpretation for the delayed effects noted by Demerec and 

 Latarjet in induced resistance mutations. 3 On the other hand, for the 

 study of phenotypic lag in bacteria, 4 segregation provides a far larger 

 and more reproducible source of material than mutation. 



In respect to lactose fermentation, such a lag cannot last more than a 

 few cell generations in this material, in view of the appearance of numerous 

 Lac — sectors in Lacy colonies on EMB — lactose agar. The masking effects 

 noted 5 with yeast asci segregating in the presence of the substrate have 

 thus not beeen found here. 



Cytological and single cell studies on these strains are in progress. 



Summary. — 1. Unstable prototrophs have been isolated from certain 

 crosses in E. coli K-12, and characterized as segregating heterozygous 

 diploids. 



2. The capacity to produce appreciable numbers of persistent hetero- 

 zygous diploids is inherited. 



3. The segregation of various mutant factors is strongly biased, possibly 

 due to a recessive lethal deletion including a locus affecting maltose fer- 

 mentation. 



4. In the heterozygotes, the type + alleles of factors controlling several 

 fermentations and nutritional requirements are dominant. Sensitivity to 

 bacteriophage Tl is dominant to resistance. 



* Paper No. 391 from the Department of Genetics, College of Agriculture, University 

 of Wisconsin. This work has been aided by a grant from the Wisconsin Alumni Re- 

 search Foundation. The author is indebted to Professor James F. Crow for his stimulat- 

 ing discussions 



1 Tatum, E. L., and Lederberg, J., /. Bad., 53, 673-684 (1947). Lederberg, J.. 

 Genetics, 32, 505-525 (1947). 



2 The composition of EMS-, in grams per liter, is: Sodium Succinate 5; (NHO2SO4 

 5; NaCl 1; MgS0 4 1; K 2 HP0 4 2; Agar 15; Methylene Blue Hydrochloride 0.065; 

 Eosin Y 0.4. This formula is conveniently stored as a dry mixture of the powdered 

 components. 



s Demerec, M., and Latarjet, R., Cold Spring Harbor Symposia Quant. Biol., 11, 

 38-50 (1946). 



4 Newcombe, H. B., Genetics, 33, 447-476 (1948). 



6 Spiegelman, S., Lindegren, C. C, and Lindegren, G., these Proceedings, 31, 95-102 

 (1945). 



170 



